2006 Volume 40 Issue 1 Pages 15-31
The southern Central India craton (also known as Bastar craton) has experienced several episodes of mafic magmatism during the early Precambrian time. These mafic igneous rocks are emplaced as dyke and volcanic rocks. A set of mafic dyke swarm and few exposures of mafic volcanics, both emplaced in the Neoarchaean time, contain high-SiO2 (>52 wt%), high-MgO (>8 wt%), and low-TiO2 (mostly <0.5 wt%) and classified as boninites on IUGS classification scheme. Al2O3/CaO ratio classifies these high-Mg mafic igneous rocks into high-Ca and low-Ca (Type 3) boninites. High-Ca boninites show typical boninitic geochemical characteristics, whereas low-Ca variety shows geochemical characteristics similar to the high-Mg norites. Siliceous high-magnesium basalts (SHMB) show similar geochemical properties as observed for the high-Mg norites. Early Precambrian high-Mg norites are reported from the rift setting environment but, on the other hand, boninitic magmatism represents an enigmatic style of magmatism and commonly formed at convergent margin settings. Most boninites are Phanerozoic in age and rarely reported from the Archaean terrains. The established geological and tectonic setting of the Archaean Bastar craton clearly suggests existence of a stable continental rift environment in the region since the Archaean time. Thus, report of Neoarchaean boninite-like rocks in an intracratonic setting is an important feature noticed in the Archaean terrain of Indian peninsula. A highly refractory mantle source region is required to form boninitic magma. Refractory lithosphere may be result of extensive extraction of mafic magma during Mesoarchaean. The preservation of refractory reservoir beneath the Bastar craton for long period of time corroborates a high degree of crustal stability during the Meso-Neoarchaean time. Trace element modelling suggests that high-Mg mafic igneous rocks under study are product of different pulses of boninitic magma produced by ∼20-25% melting of a refractory mantle source.
